A solid state drive (SSD) is a data storage device that uses integrated circuit assemblies as memory to store data persistently. SSDs have no moving mechanical components and this distinguishes SSDs from traditional electromechanical magnetic disks, such as, hard disk drives (HDDs) or floppy disks, which contain spinning disks and movable read/write heads. Compared to electromechanical disks, SSDs are typically more resistant to physical shock, run silently, have lower access time, and less latency. Many types of SSDs use NAND-based flash memory which comprises an electronic (solid-state) non-volatile computer storage medium that can be electrically erased and reprogrammed.
SSDs are different from HDDs in how they wear in comparison to HDDs. SSDs have a service lifetime that is a function of the number of write operations (known as program/erase cycles) that the NAND-based flash memory is able to endure. Techniques for wear leveling may be used to prolong the service lifetime of SSDs. In certain techniques for wear leveling, erasures and re-writes are distributed evenly across the NAND-based flash memory of the SSD, via mechanisms such as background data movements. In this way, no single erase block of the NAND-based flash memory prematurely fails due to a high concentration of write cycles.
The uncorrectable bit error rate (UBER) for a SSD is the number of data errors per number of bits read. The UBER for a SSD may increase with the age of the SSD.